The present invention relates to a copier, facsimile transceiver or similar image forming apparatus and, more particularly, to a charging arrangement capable of preventing a toner from depositing on non-charged part of an image carrier at the start of an image forming operation.
An electrophotographic image forming apparatus, e.g., an electrophotographic copier charges the surface of a photoconductive element or similar image carrier by a charging unit, exposes the charged surface of the image carrier to imagewise light to form an electrostatic latent image, and develops the latent image by a developer, e.g., a toner stored in a developing unit to form a toner image. The problem with this type of apparatus is that at least part of the surface of the image carrier between the charging unit and the developing unit is not charged just after the start of an image forming operation. When such part of the image carrier passes the developing unit, the toner stored in the developing unit unavoidably deposits on the non-charged part even if a bias for development has been switched to zero beforehand. The toner on the drum is apt to increase the load on a cleaning unit to thereby reduce its life. To eliminate this problem, it is a common practice to connect a constant voltage generation circuit constituted by a Zener diode and a capacitor to the casing of the charging unit via a varistor and to connect the image carrier to ground via the Zener diode. In this configuration, simultaneously with the start of an image forming operation, the constant voltage generation circuit causes a current to flow through the image carrier. As a result, the image carrier is biased to prevent the toner from undesirably depositing thereon.
On the other hand, a contact charging system is known in the art which connects a charging roller to a charge power source and holds it in contact with the image carrier. This kind of scheme noticeably reduces the current required to flow through the image carrier via the charging roller. However, since the image carrier is simply connected to ground via the Zener diode, the current is too small to generate a Zener voltage. Hence, it is difficult to develop an electric field intense enough to eliminate the toner deposition stated above.